A tank belonging to the Libyan National Army, led by Khalifa Haftar, last year.
Photograph: Abdullah Doma/AFP/Getty Images

An attack on an airbase in southern Libya has killed 141 people, mostly soldiers loyal to military strongman Khalifa Haftar, according to a spokesman for his forces.

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On Thursday, members of the Third Force militia loyal to the UN-backed government of national accord (GNA) in Tripoli attacked the base used by Haftar’s self-proclaimed Libyan National Army (LNA), according to military sources.

An LNA spokesman, Ahmad al-Mesmari, said that the victims included civilians who worked at the Brak al-Shati airbase or who were in the nearby area. “The soldiers were returning from a military parade. They weren’t armed. Most of them were executed,” he said.

The GNA said that a commission of inquiry had been set up to investigate the attack. It said it had decided to suspend its defence minister, Al-Mahdi al-Barghati, and the head of the Third Force from their duties until those responsible were identified.

The GNA and defence ministry condemned the assault and said they had not ordered any such action.

The LNA does not recognise the authority of the GNA and instead supports the rival authorities based in the east.

The UN envoy to Libya voiced alarm at reports of the attack on the base, 650km (400 miles) south of Tripoli. “I am outraged by reports of significant numbers of fatalities, including civilians and by reports that summary executions may have taken place,” the UN envoy, Martin Kobler, said.

Britain’s ambassador to Libya, Peter Millett, also denounced the assault. “Disgusted by attack on Brak al-Shati & reports of mass executions. Perpetrators must be brought to justice,” he wrote on Twitter.

The incident at the base in Wadi al-Shati district took place a month after an attack by the LNA on the Tamenhant airbase controlled by the Third Force near the south’s main city of Sebha.

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That attack was called off after a reconciliation meeting between Haftar and the head of the unity government, Fayez al-Sarraj, in Abu Dhabi on 2 May.

The speaker of the eastern-based parliament, which is supported by the LNA, accused the Misrata-based Third Force of a “serious breach of the truce agreement reached in Abu Dhabi”. Aguila Saleh said there were a “number of martyrs” in LNA ranks, without giving a figure.

The speaker said he had given orders for the armed forces “to take the measures necessary to respond to the assault and defend the south and cleanse it of all outlaw militias”.

In Tripoli, the GNA called for an immediate ceasefire in the south where tribes and militias vie for control of lucrative smuggling routes with neighbouring Chad, Niger and Sudan. “We hope reason will prevail and that the escalation and provocation will stop,” it said.

The unity government’s defence ministry laid the blame on “those who started bombing Tamenhant base with warplanes and tanks,” referring to Haftar’s forces.

Elsewhere on Friday a pro-Haftar tribal chief, Sheikh Ibrayek Alwati, and five other people including a child were killed in a car bombing outside a mosque in the eastern city of Slouq, according to medical and security sources.